1. Field of the Invention
The present invention relates to a method and an apparatus for analyzing trace impurities in gases, and particularly to a method and an apparatus for detecting ppbxe2x80x94sub ppb level of trace impurities in various high-purified gases, by a combined analyzer comprised of gas chromatography and atmospheric pressure ionization mass spectrometer.
2. Description of the Prior Art
In case of analyzing (detecting) the impurities in high-purified gases by combined analyzer comprised of gas chromatography and atmospheric pressure ionization mass spectrometer, the outflowed gas(30-50 cc/min) from a gas chromatography which uses packed column, does not reach the gas flow(100-500 cc/min) necessary for a common atmospheric pressure ionization mass a spectrometer. Therefore, the outflowed gas from a gas chromatography was added by purified gases, the kind of which is the same with the carrier gas used in the gas chromatography, and then introduced into an atmospheric pressure ionization mass spectrometer(Japanese Patent Laid Open Gazette Hei. 6-3461 Japanese Patent Laid Open Gazette Hei.9-15207)
High-purified He(helium) or Ar gas is commonly used as the carrier gas and the purified gas. In particular, He is much preferred, because its ionization potential (24.59 eV) is higher than those of other gases, and all kinds of impurities except He can be detected.
Nevertheless, when He gas is used as a carrier gas, it is also sensitive to some main component such as O2, N2 and Ar, and their ions are generated in a large quantities. Therefore, some impurities hardly separable from the main components could not be detected well, and the measurement with a high sensitivity was hard to be obtained.
Under the circumstance, a method, in which impurities and main components are separated before being introduced into the atmospheric pressure ionization mass spectrometer, has been carried out to improved the sensitivity of measurement. For example, a complicated applied flow passage using a method such as xe2x80x9cHeart Cut methodxe2x80x9d, is installed and then cut just before the main component is outflowed from a gas chromatography, and the outflowed gas is introduced into the atmospheric pressure ionization mass spectrometer about at the point of time when the outflow of the main component ends.
However, there is a problem in the above method, in that the main components near the impurities were not separated well, and the introduction of the main components into the atmospheric pressure ionization mass spectrometer could not be avoided completely, which causes increased noises and weakened sensitivity when analyzing the impurities.
There is another problem that as columns used for a long-term could increase the retention time of the main components and thus the subject impurities could be separated and removed. Therefore, the retention time should be checked regularly, which requires much labors, and many columns for removing the main components and complicated flow are also required.
There is still another problem in another method of removing the main components by an absorbent, in that the subject impurities are removed together with the main components, or the other impurities are generated from the absorbent in ppb-ppm levels, resulting that the measurement in ppb level is hard to perform.
In addition, in case of using a atmospheric pressure ionization mass spectrometer, as He generates helium cluster ion (He4+)(mass number=16) in a large amount, CH3+(mass number=15) is measured for analyzing methane(M.W.=16). However, there is a problem that the sensitivity of mass number 15 is not so good as mass number 16(CH4+). Moreover, as the discharge of He is less stable compared with other gases and the stability of the main components are not good, the sensitivity to H2, which is detected by He2H+(mass number=9) generated by combining He2+ and proton, is also bad.
In case of Ar, which has a good discharge nature, there is a problem that the ionization potential of Ar(15.76 eV) is near that of N2 (15.58 eV), and thus transfer of electric charges is hardly generated and the measurement of the impurities like N2 or Ne, whose ionization potentials are higher than those of Ar, is impossible.
Under the circumstance, when the impurities are measured to the level of sub ppb in high purified O2, He is used for Ne or N2 of impurities, Ar is used for of H2, CH4, CO or CO2 of impurities. That is, He should be supplied for analyzing N2 etc., and Ar should be supplied for analyzing H2 etc., as a carrier gas and purified gases added. Therefore, much labor and time were required for switching the supply of the gases, and the measurement cannot be carried out quickly.
The other method has been proposed, in which a third component gas is incorporated into the sample gas when the analysis is performed by atmospheric pressure ionization mass spectrometer alone (Japanese Patent Laid-Open Gazette Hei.6-7490). However, the method has a problem that the impurities in the third component gas should be checked in advance, because they could have an effect on the analysis of the impurities in the sample gas.
For example, when analyzing N2 as an impurity in Ar, as the ionization potentials of both gases are mutually near each other, the sensitivity of the analysis is very bad and the measurement cannot be carried out at ppb level. Therefore, a method has been proposed, in which the measurement is performed by an atmospheric pressure ionization mass spectrometer using proton transfer rection after adding H2 in the level of % into the sample gas of Ar. In this method, the check of the impurity N2 in H2 added, or the separation of CO in Ar both of which have the same mass number is hardly performed. Therefore, the measurement is carried out for N2+CO, and thus the concentration of CO in the sample gas should be checked in advance, resulting in a very tedious and bothering work.
The present invention is purposed to provide a method and an apparatus for analyzing trace impurities in gases, which enable to analyze a very small quantity of impurities by a simple operation, without making the column arrangement or the structure of flow complicated.
In accordance with the object of the present invention, there is provided a method for analyzing the trace impurities in gases, the method comprising the steps of:
separating main components and trace impurities from a sample gas conveyed by a carrier gas by gas chromatography;
introducing the gases outflowed from said gas chromatography into an atmospheric pressure ionization mass spectrometer; and
analyzing the trace impurities, wherein a mixed gas is used as the carrier gas.
In accordance with another object of the present invention, there is provided a method of analyzing the trace impurities in gases, the method comprising the steps of:
separating main components and trace impurities from a sample gas conveyed by a carrier gas by gas chromatography;
adding purified gas into the gases outflowed from said gas chromatography; introducing the gases into an atmospheric pressure ionization mass spectrometer; and
analyzing the trace impurities, wherein a mixed gas is used as at least one of the carrier gas and the purified gas.
In accordance with still another object of the present invention, there is provided a method of analyzing the trace impurities in gases, the method comprising the steps of:
separating main components and trace impurities from a sample gas conveyed by a carrier gas by gas chromatography;
introducing the gases outflowed from said gas chromatography into an atmospheric pressure ionization mass spectrometer; and
analyzing the trace impurities, wherein a single component gas is used as the carrier gas, and purified gas which are different from the carrier gas, are added to the outflowed gas.
In the above case, the method is characterized in that if said carrier gas is He, the added purified gas is Ar alone, or Hexe2x80x94Ar mixed gas, or Hexe2x80x94H2 or Arxe2x80x94H2 mixed gas, and if said carrier gas is Ar, the added purified gas is He alone, or Hexe2x80x94Ar mixed gas.
In accordance with still another object of the present invention, there is provided a method of analyzing the trace impurities in gases, the method comprising the steps of:
introducing a sample gas into a gas chromatography using He as a carrier gas;
separating main components and trace impurities from the sample gas;
adding a purified gas of Ar alone or Arxe2x80x94He mixed gas into the gases outflowed from the gas chromatography;
introducing the gases into an atmospheric pressure ionization mass spectrometer; and
analyzing the trace impurities, wherein the impurity of H2 is detected by mass number 41 or 81, and the impurity of methane is detected by mass number 16.
In accordance with still further object of the present invention, there is provided a method of analyzing the trace impurities in gases, the method comprising the steps of:
introducing a sample gas into a gas chromatography using He as a carrier gas;
separating main components and trace impurities from the sample gas;
adding a purified gas into the gases outflowed from the gas chromatography;
introducing the gases into an atmospheric pressure ionization mass spectrometer; and
analyzing the trace impurities, wherein at least two purified gases selected from i) He alone, ii) Hexe2x80x94Ar mixed gas and iii) Hexe2x80x94H2 mixed gas or Arxe2x80x94H2 mixed gas, are selected and switchably used as said purified gas.
In accordance with still further object of the present invention, there is provided an apparatus for analyzing the trace impurities in gas, the apparatus comprising:
a gas chromatography for separating the main component and trace impurities from the sample gas conveyed by carrier gas;
an atmospheric pressure ionization mass spectrometer connected to the back part of the gas chromatography; and
a purified gas adding passage for adding the purified gas outflowed from the gas chromatography, the purified gas adding passage being installed in a passage between the gas escaping part of the gas chromatography and the gas introduction passage of the atmospheric pressure ionization mass spectrometer,
wherein i) a passage for supplying a purified gas whose kind is the same as the carrier gas, ii) a passage for supplying a purified gas whose kind is different from the carrier gas, and iii) mixed ratio regulating means installed in the purified gas adding passage for regulating a mixed ratio of both purified gases, and added amount controlling means installed in the purified gas adding passage for regulating the added amount of purified gases depending on the kind of the outflowed gas are further provided.